Difference between ISO 14644 and ASHRAE standards
In cleanroom design, the choice of standard is a key factor to ensure effective airflow control. This article analyzes the differences between ISO 14644 and ASHRAE standards, helping businesses and engineers understand the differences in air cleanliness classification, ventilation design, air velocity and air supply efficiency.
I. OVERVIEW OF ISO 14644 AND ASHRAE
In the field of cleanroom design and operation, understanding and correctly applying airflow control standards is fundamental to ensuring performance and compliance with regulatory requirements. Two prominent standards today are ISO 14644 and ASHRAE - each system offers a different approach, serving specific objectives and industries. Before going into detailed comparisons, we need to clarify the role and scope of each standard.
1. What is ISO 14644?
ISO 14644 is an international standard specifically for classifying and controlling airborne cleanliness in cleanrooms, based on particle concentration in the air. It was developed by the International Organization for Standardization (ISO) and is widely applied in industries requiring ultra-clean environments such as pharmaceuticals, microelectronics, semiconductor manufacturing, and biotechnology.
Key parts of ISO 14644 include:
- ISO 14644-1: Defines cleanroom classification from Class 1 to Class 9 based on particle concentration at sizes ≥0.1μm to ≥5.0μm.
- ISO 14644-3: Provides testing methods for cleanroom performance, such as airflow measurement, air velocity, and HEPA filter leakage tests.
- ISO 14644-4: Offers guidance on cleanroom design, construction, and layout according to required cleanliness levels.
- ISO 14644-5: Provides guidelines for operation and maintenance, including cleaning procedures and periodic checks.
ISO 14644 is particularly important in GMP (Good Manufacturing Practice) facilities, where the presence of particles, microorganisms, or contaminants can seriously impact product quality.
2. What is ASHRAE?
ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) is a U.S.-based association of engineers that develops standards and guidelines for HVAC (Heating, Ventilation, and Air Conditioning). Unlike ISO 14644, which focuses on cleanliness classification, ASHRAE emphasizes HVAC system design and operation to ensure proper ventilation and air distribution, including for cleanroom applications.
Common ASHRAE standards include:
- ASHRAE Standard 170: Ventilation requirements for healthcare facilities - considered the benchmark for hospital design, operating rooms, and ICUs.
- ASHRAE Handbook (HVAC Applications): Provides technical specifications and recommendations for airflow rates, velocity, and pressure balancing in various facilities - from offices to laboratories.
- ASHRAE Standard 62.1: Ventilation standards for indoor air quality in residential and commercial buildings.
ASHRAE focuses on efficient HVAC design and operation, ensuring proper air supply and return, pressure control, temperature, and humidity management in the built environment.
3. Differences in application scope
The table below summarizes the main differences between ISO 14644 and ASHRAE:
| Criteria |
ISO 14644 |
ASHRAE |
|---|---|---|
| Main objective |
Air cleanliness classification by particle concentration |
HVAC design and operation guidelines |
| Application |
International, mainly pharmaceuticals, electronics, high-tech industries |
Mainly U.S., strong focus on healthcare and medical facilities |
| Measurement |
Particle counts ≥0.1μm/m³ |
CFM (Cubic Feet per Minute), ACH (Air Changes per Hour), airflow velocity (m/s) |
Choosing between ISO 14644 and ASHRAE - or combining both - depends on the field of application, product requirements, and legal regulations in each country. The following section compares in detail the requirements for airflow, pressure, ACH, and velocity between the two systems.
See more: Application of AI in the Pass Box System for Cleanrooms
II. DETAILED COMPARISON OF AIRFLOW REQUIREMENTS BETWEEN ISO 14644 AND ASHRAE
In cleanroom design and operation, airflow is not only a technical factor but also the core element for product quality, safety, and international compliance. Both ISO 14644 and
ASHRAE provide requirements and recommendations for airflow velocity, flow patterns, air changes per hour, and pressure - but they are built on different philosophies and objectives.
1. Cleanliness classification: ISO quantitative - ASHRAE qualitative by application
ISO 14644-1 defines cleanroom classes by airborne particle concentration, from Class 1 to Class 9. Each level specifies limits for particles ≥0.1μm, ≥0.5μm, ≥5.0μm/m³.
ASHRAE, especially in Standard 170, does not classify by particle concentration. Instead, it sets ventilation and filtration requirements for specific room types: operating rooms,
ICUs, laboratories, etc. Each has its own airflow and HEPA filtration requirements.
Conclusion: ISO uses quantitative particle-based classification; ASHRAE relies on room function and control level.
2. Air velocity and Air Changes per Hour (ACH)
ACH is one of the most critical design parameters. Both systems provide guidance, but with clear differences:
| Cleanroom (ISO) |
ISO 14644 - Typical ACH |
ASHRAE - Recommended ACH |
|---|---|---|
| ISO Class 5 |
240-480 ACH |
~300 ACH (Class B operating rooms) |
| ISO Class 7 |
60-90 ACH |
20-30 ACH |
| ISO Class 8 |
10-30 ACH |
6-15 ACH |
Explanation:
ISO does not directly prescribe ACH but relies on industry guidelines (GMP, PIC/S, FDA), which link ACH values to cleanliness classes.ASHRAE calculates ACH based on HVAC system capacity, room size, and activity type.
Key difference: ISO sets particle limits; ASHRAE defines HVAC parameters to achieve them.
3. Airflow type: unidirectional vs non-unidirectiona
ISO 14644 requires unidirectional airflow (UDAF) for Class 5 or higher in critical zones (e.g., weighing, filling). Air must flow in a uniform direction, usually vertical from ceiling to floor, at sufficient velocity to sweep particles away.
ASHRAE allows flexibility: unidirectional (e.g., certain operating rooms), non-unidirectional (mixed airflow), or hybrid systems.
Difference: ISO mandates UDAF at higher classes; ASHRAE accepts different airflow types if performance is achieved.
4. Pressure differentials and zoning
ISO 14644 (with GMP) recommends maintaining 10-15 Pa differential between adjacent zones of different cleanliness. Continuous monitoring and alarms are required.
ASHRAE 170 specifies room-based pressure values, e.g.:
- Operating rooms: +2.5 Pa relative to corridors
- Isolation rooms: -2.5 Pa relative to outside
- Biosafety labs: typically -10 Pa or more
Difference: ISO applies general differential values; ASHRAE details per room type, with emphasis on healthcare.
5. Face velocity at filters
| Factor |
ISO 14644 |
ASHRAE (170/Handbook) |
|---|---|---|
| Cleanliness class | By particle concentration ≥0.1μm |
By room function and type |
| ACH | High (up to 480 ACH at Class 5) |
Medium-low (6-30 ACH depending on room type) |
| Airflow | Mandatory unidirectional at higher classes |
Flexible: unidirectional, non-unidirectional, or hybrid |
| Pressure | 10-15 Pa between zones |
Specific pressure values per room type |
| Air velocity | At HEPA filter face: 0.45 m/s ± 20% |
At supply diffuser, depending on application (0.13-0.5 m/s) |
| Preferred use | Pharmaceuticals, electronics, microbiology |
Hospitals, operating rooms, healthcare facilities, laboratories |
See more: How to Choose the Right Cleanroom Standard System by Industry
III. APPLICATIONS IN CLEANROOM DESIGN
The choice between ISO 14644 and ASHRAE in cleanroom design is not only a matter of technical parameters but also depends on industry regulations, intended use, and the desired level of environmental control. In practice, each sector has specific requirements, and designers and operators must understand them to apply the correct standards - or combine both - to achieve optimal efficiency.
1. Pharmaceutical industry (GMP)
In pharmaceutical manufacturing, controlling particulate and microbiological contamination is mandatory to ensure product and patient safety. GMP guidelines (EU-GMP, WHO-GMP, PIC/S) require the use of ISO 14644 as the basis for classifying and controlling cleanroom levels.
ISO 14644 is used to:
- Classify areas such as weighing, filling, and packaging rooms (typically Class 5-8)
- Perform routine airborne particle counts
- Ensure unidirectional airflow in critical zones
ASHRAE, while not officially recognized for GMP compliance, can be used as a reference to:
- Design optimized HVAC systems in terms of air velocity, airflow rates, and fresh air supply
- Control temperature and humidity for product protection and storage
Conclusion: ISO 14644 is mandatory in pharmaceuticals, while ASHRAE supports HVAC system design to meet compliance.
2. Electronics and semiconductors
In microchip, display, and electronic device manufacturing, ultra-clean environments are required to prevent product defects caused by microscopic dust or electrostatic discharge. This is one of the sectors that strictly applies ISO 14644-1, especially Classes 1-5, where particle limits are extremely low.
Key requirements include:
- High-velocity unidirectional airflow (0.45-0.6 m/s)
- HEPA or ULPA filtration at the highest levels (99.999%)
- Stable pressure differentials between adjacent areas
ASHRAE is rarely applied in this industry since it does not provide sufficient detail for ultra-clean requirements.
Conclusion: ISO 14644 is the core standard in cleanroom design for electronics and semiconductors.
3. Healthcare and hospitals
Unlike pharmaceuticals and electronics, in healthcare - particularly hospitals, operating rooms, and ICUs - ASHRAE Standard 170 is the primary recognized standard.
ASHRAE 170 specifies:
- Ventilation design for different room types (operating rooms, labs, isolation rooms, etc.)
- Minimum air changes per hour (ACH), e.g., operating rooms (Class B) require ≥20 ACH
- Positive/negative pressure differentials depending on infection control needs
ISO 14644 may be referenced in cases where hospitals require specialized cleanrooms such as:
- Oncology drug preparation rooms (ISO Class 5)
- Molecular biology laboratories
Conclusion: ASHRAE 170 is mandatory for healthcare HVAC design, while ISO is applied only in specialized clean zones.
4. When to combine both standards
In complex projects - such as pharmaceutical plants with multiple production lines or multi-disciplinary research centers - combining ISO 14644 and ASHRAE is often the best approach.
Effective integration approach:
- Use ISO 14644 to define the “goal”: required cleanroom class (Class 5, 7, 8, etc.)
- Use ASHRAE to design the “means”: HVAC system with appropriate airflow, ACH, and pressure to meet the target
Example:
A weighing room requires ISO Class 7 → design with ≥60 ACH, HEPA filtration, and 15 Pa pressure differential.
To achieve this, ASHRAE 170 can be referenced to calculate supply and return airflows, diffuser layouts, and ceiling-to-floor air distribution.
Conclusion: ISO defines the cleanliness outcome, while ASHRAE provides the design tools to achieve it.
IV. SUMMARY TABLE OF ISO 14644 VS. ASHRAE
The table below summarizes the key differences between ISO 14644 and ASHRAE in cleanroom design and operation. This content is ideal for use in FAQs or as a short summary on industry-specific landing pages:
| Criteria | ISO 14644 | ASHRAE (Standard 170/Handbook) |
|---|---|---|
| Cleanroom classification |
Based on airborne particle concentration ≥0.1μm/m³ |
Based on room type and function |
| Recommended ACH |
High - 10 to >400 air changes/hour depending on class |
Moderate to low - typically 6-30 air changes/hour |
| Airflow pattern |
Mandatory unidirectional airflow for Class 5 and above |
Flexible: unidirectional, mixed, or hybrid |
| Pressure control |
Recommends 10-15 Pa differential between zones |
Specifies exact pressure values depending on room type |
| Air velocity measurement |
At HEPA/ULPA filter face |
At supply diffuser |
| Target industries |
Pharmaceuticals, electronics, microbiology, semiconductors |
Healthcare, hospitals, clinics, general HVAC applications |
| Role in design |
Defines cleanroom class (air quality target) |
Provides HVAC design methods to achieve compliance |
See more: Differences between P1, P2, P3, and P4 laboratories based on biosafety standards
V. CONCLUSION AND RECOMMENDATION
- ISO 14644: Used to define the target cleanroom class (Class 5, 7, 8, etc.)
- ASHRAE: Used to design an HVAC system that achieves the defined target efficiently and sustainably
